Great start, but one important factor is missing from this analysis: The surface area (for heat exchange) of a closed-loop borehole is tiny compared to the surface area in a natural hydrothermal system. The walls of the borehole will cool off relatively quickly due to the low thermal conductivity of rock, and you'll soon be unable to make power. Conventional geothermal gets around this by pulling from very large reservoirs that include natural convection and huge fracture surface area.

This means that a closed loop system needs many many many miles of boreholes in order to last long enough to pay off.